Renovation
A tighter house needs deliberate ventilation
Old buildings ventilated themselves through gaps nobody designed. Sealing those gaps saves energy and removes the accidental air change that was keeping the house dry.
This works through ventilation strategy in the order the parts actually depend on each other.
The short version
- Draughtproofing removes uncontrolled air change, which has to be replaced deliberately.
- A household produces moisture continuously from breathing, washing and cooking.
- Fuel-burning appliances need air supply and are a matter for a qualified professional.
What draughts were doing
A leaky building exchanges its air constantly through gaps at floors, windows, chimneys and service penetrations. That exchange is wasteful of heat and uncontrolled, arriving where the wind decides rather than where it is needed.
It is also, unintentionally, what kept many older houses free of condensation and mould for a long time. Sealing a building without adding controlled ventilation removes the mechanism and leaves the moisture load in place. This is why homes sometimes develop condensation problems shortly after new windows, insulation or draughtproofing are installed.
The moisture and pollutant load
People release water vapour continuously through breathing and perspiration, and a household adds much more through washing, cooking and drying. Carbon dioxide accumulates in occupied rooms, particularly bedrooms overnight with the door closed and the window shut. Furnishings, finishes and cleaning products release volatile compounds, and the concentration depends on how often the air is changed.
Underneath it, cooking produces particles and moisture together, which is why extraction at the hob does more than any general ventilation. The general principle is to remove pollutants at their source and to provide background air change everywhere else.
Strategies, from simplest to most involved
Background ventilation through trickle vents and passive openings provides a continuous small exchange without any energy input. Intermittent extract fans in kitchens and bathrooms deal with the peaks of moisture where and when they are produced.
Continuous mechanical extract runs at a low rate all the time and boosts when needed, which gives more consistent results. Systems with heat recovery extract stale air and use its warmth to temper incoming fresh air, which suits well-sealed buildings. Each of these needs designing for the specific house, and a system installed without commissioning rarely performs as intended.
Where it goes wrong in practice
Fans that discharge into a loft or a void move the moisture into the structure, which is worse than leaving it in the room. Undersized or over-long ducting throttles a fan so that it makes noise and moves very little air.
The quieter point is that occupants switch off systems they find noisy or do not understand, which returns the house to the sealed condition. Blocking trickle vents in cold weather is extremely common and is a reliable way to produce condensation on the same windows.
Internal doors with no undercut prevent replacement air reaching an extract fan, which makes the whole arrangement ineffective.
Ventilation and insulation together
Insulation raises internal surface temperatures, which reduces condensation risk on those surfaces, so the two measures work together. Insulating unevenly can create cold spots at junctions, and condensation then concentrates on those remaining cold surfaces.
Live with it a while and insulating an old solid wall changes how moisture moves through it, which is a technical question with real consequences for the fabric. The order matters: planning ventilation at the same time as insulation avoids the sequence where sealing precedes provision. Where a building has traditional construction, specialist advice is worth obtaining because the general modern approach may not suit it.
Taste is not the argument here; how a room performs is.
The safety boundary
Any appliance that burns fuel needs a supply of air and a route for its products, and starving one is dangerous rather than merely inefficient. Extract fans can create pressure differences that affect open-flued appliances, which is why the combination requires professional assessment.
The quieter point is that carbon monoxide is odourless, and detectors are inexpensive and recommended in most countries wherever fuel is burned. Rules covering ventilation rates, appliance clearances and who may carry out the work differ considerably between countries. For anything involving gas, solid fuel or flues, the correct step is to engage a qualified professional rather than to follow general guidance.
The takeaway
If you seal a house, give it a deliberate way to breathe, and keep anything that burns fuel firmly in professional hands.
A room that is used will always beat a room that is finished.
Questions readers ask
Why did condensation start after I got new windows?
The old windows leaked air continuously and kept humidity down. New sealed units remove that accidental ventilation, so the moisture the household produces has nowhere to go.
Should I keep trickle vents open in winter?
Generally yes. They provide the background air change that keeps humidity down, and closing them is a common cause of condensation on the very windows they are fitted to.
